The Unknown Sister
by silverfox56
Summary: What if the TMNT had a sister? I know this isn't a very good summary. Honestly, I can't think of anything decent.
1. Prologue

_Hi there everyone! This is my first fanfiction ever! I'm really excited to share it with you guys. Like I said, this is my first, so I'm open to constructive criticism for how to make it better. I'm also looking for some ideas for titles. I put one in that I kind of like, but I don't LOVE it. If you have any suggestions, I'd love to here them. Also, if this doesn't get many reads, then I'm not going to post more. I don't want to spend my time doing this if no one is enjoying it. Sorry. Oh, and this story is rated M._

_Hope you enjoy._

**Prologue**

This is my story. A side to one story that you've never heard before. You've all heard of the famous Teenage Mutant Ninja Turtles. The ones that save the city and the world from threats like the Kraang and Shredder. But you've never heard of one of their greatest secrets. The secret that they want to keep hidden from everyone, especially their most hated enemy.

And that secret would be me. Their little sister. I haven't been part of their family long, only 2 years, and I'm the outcast. They try to include me in everything they do, except going to the surface. They never let me leave home and I don't feel like leaving. This is the first real home that I've had. The hole I lived in for half of my life doesn't count. Now I bet you're wondering how I came to be. It's kind of a long story, so I'll try to shorten it.

Everyone at TCRI was obsessed about how the mutagen affected the turtles and wanted to learn more. So they tried to make another one. Somehow, they got their hands on a lot of mutagen. When I was locked up, I saw an entire room full of it.

They bred many Red-Eared Sliders and splashed mutagen on them when they became infants. I was the first successful mutant that didn't have any side-effects. I can only imagine the horrors that came upon the ones that weren't so lucky.

I was under lock and key for 5 years. Studied in every aspect the "scientists" could think of. My cell production, digestion, brainwaves. They hooked me up to machines, starved me to see how long I could survive without food. The same with water. And many more experiments that I would prefer not to mention. Until, I got fed up one day with how I was treated. I broke out, caused a mass panic, and hid in the sewers. That's where I stayed, for three years, hiding from the world. During that time, I sort of trained myself how to hide. I had to hide from people who came to the sewer, and in case the people from TCRI came looking for me. I taught myself how to walk without making a sound, how to sneak around the sewers without being detected, especially in the water. I can step without making a splash. When I walk on pipes, they don't even creak.

I found the TMNT about a year and a half after I freed myself. At first, I was terrified of them. Then I realized they were just like me. I became fascinated with them; how they moved, how skilled they were with their weapons. And how much of a bond they had with each other. So I followed them whenever I could. I eventually found their lair, but didn't go much further than that. I did that for the years leading up until they saw me for the first time.

I was sleeping in my hole when I heard it. The creak of pipes giving out. I couldn't get out of my home before the ceiling came crashing down on me. I tried to free myself after waking. When I shifted though, fire shot up my ankle. I looked down. It was at an angle that wasn't normal. Great, it's broken. No matter how much I tried to get myself unstuck, I couldn't get myself. I had no choice but to lay there.

They found me a couple days later, near death. I overheard Don say I was dehydrated and malnourished. I recovered fast, faster than I thought I would. I didn't know why, and I didn't care. I was just happy I wasn't dead.

When I finally met Don's health seal of approval, they tried to ask me questions about where I came from; how I ended up stuck under 300 pounds of solid rock, and anything and everything that came to their minds. But I didn't answer.

I learned how to talk by listening to the scientists who were around me 24/7. I never talked to them though. What was the point? I had nothing to say to them. Also, I didn't really trust them. Sure they saved me, but I couldn't be 100% sure that they did it out of kindness or they wanted something out of me.

So that's how it was for the next week. Them asking a lot of questions, and me not answering. Me trying to find every possible hiding spot that I was sure they would never find me in. Everytime I thought I found one, Mikey, which I guessed at the time was the youngest and the most immature, would show up. I eventually found one, high up in the wall, that looked like it had never been touched. And I was right. Every time I thought Mikey would show up, he didn't.

After a month, it was safe to say I was part of the family. I was treated like one of them, and as a living thing, not a successful experiment. This was also the time that they decided to see if I had the potential for ninjutsu. Leo brought me into the dojo, where Splinter and the rest of my "brothers" were kneeling, all facing me. Leo sat me down and went and stood next to Splinter.

"Do not worry, my child, you are not in trouble. We have all come to a decision about how to progress further. Leonardo will explain." This scared me even more. How to progress further? Were they finally going to kick me out? I studied their faces. None of them had any look or any physical indication that that's what Splinter was referring to. I looked back at Leo. he had a soft expression on his face, so I knew nothing bad was going to happen.

"As you know, young one, you have been with us for about a month. We treat you as a sister and would like to continue to have you as part of our family. Do you agree?" I stared at them and thought for a little. I finally have a family. One that treated me well. And they could protect me from my previous captores if they ever come for me again. So I nodded.

"Good. Then comes the tricky part. We have to come up with a name for you." That part I didn't expect. A name? I've never had a name. Why would I need one? I always expected that they would keep calling me "young one" and "little sis". Splinter interjects. "I might have a solution for this problem." He grabs a book out of his robe. All four of my brother gasp. Seems this book means something to them.

"Is that..." Mikey trails off. "It is my son," Splinter responds. "It is the book that I found the names for all four of you. And since it brought such luck to us before, it might bring us luck once again."

He waved me over and I sit next to him. He opens the book and flips to the female section. "When you see a name that you like, point it out, and we will go from there." He slowly turns the pages, giving me time to study every name, what it means, and what that woman did. He continued to flip, and I continued to read, but I couldn't find one that I liked. He finally stopped. "This is the last page, young one. I hope you can find a name on this one."

He turned it over and I looked. It was the last woman on the page. She was tucked away in the corner, so little writing next to their picture you might have skipped over it. But I didn't. She wasn't the most beautiful woman, but I loved her name. I stuck my finger at her picture and Splinter learned over to look at it. He smiled. "I like that name. It suits you."

Everyone else leaned over to look at the name. After reading it, everyone agreed. Except Raph. he didn't say anything, and knowing him, it probably meant he didn't like it and was expecting more of a badass name.

I finally had a name. An identity. I was jumping up and down inside, but showed no emotion on the outside. What was the point? They already knew I liked it.

"Catharina it is," said Leo. He smiled. "Now that we have that dealt with," began Splinter, "we had one other thought." he looked down at me. "Would you like to learn ninjutsu?" My mouth fell open, but I snapped it shut. I would have to think about this one. So I shrugged. "We'll give you some time to think about it, of course," Donnie butted in. "You have to let us know soon so we can start. You're a far way behind the rest of us."

I sat there thinking about it, so deep in my thoughts, I didn't notice any of them leave the dojo. Soon I was the only one there, sitting in the middle of the room, staring at the floor. Do I want to learn? It would be cool to learn. And I would be able to protect myself if the TCRI guys ever came after me. But could I ever live up to the rest of them? They've been doing this much longer than I have and have way more experience. Would I ever be as good as them? I must have sat there for hours because the place was quiet. Everyone must have already gone to bed.

I walked over to the couch , where my makeshift bed was. It was mine until they could find another hole in the sewer that wasn't being used. Donnie offered to move his lab, but I shook my head. He loved that place. It was his private little area. I wouldn't take that away from him. I laid down on it and pulled the covers up to my chin and fell asleep straight away.

A week passed before I agreed to start training. Turns out. I was a natural. Even more than Mikey. I mastered everything physical they had learned in 16 years in less than a week. Then Splinter let me spar with them. I'm pretty sure Leo was jealous when I beat him in the first minute.

After that, we started with the weapons. I mastered those in two. I surpassed them in that field as well. Now for this, I'm sure Raph was pissed. I could see the anger in his eyes. He didn't act on it though. That made me happy.

Now that you know most of my backstory, I can finally move on.


	2. Chapter 1

**Hey people of the internet! I saw that some people were interested in this story after all, but I don't believe that I can judge this story's popularity from one chapter, so I'm going to add another one. Once again, constructive criticism on this story (spelling mistakes, grammar, scenes that are relevant or irrelevant) is greatly appreciated. I've also not been able to think of a better title for this story, so if any of you have an ideas, let me know! I don't remember if I put this in the previous doc, so I'm going to say it here, and only once. I DO NOT OWN TMNT OR ANYTHING OTHER THAN MY OC! Hope you enjoy!**

I woke up early in the morning in my secret spot. I yawned and stretched, feeling way too tired to be up at this time. I looked at the clock that I made. 5 am. I still keep replaying Donatello's stunned expression when I made it on my first try. He demanded a competition to see who could build one the fastest. Everyone warned him that it would be a bad idea. It was. I build mine in less than 5 minutes, and he took twice the time. I really didn't want to show him up. This was his gift after all, so I wordlessly told everyone to never make me do that again.

I crawled out of my "room" and went to the kitchen. I got out the pancake batter and a bowl to mix it in. Then the bacon from the fridge I made pancakes and bacon for my brothers. I also put on a pot of coffee for Donnie. Either he's still up in his lab or he's only gotten a few hours of sleep. Me and him are having a race to see who can function without sleep without even knowing it. I eat my part of the breakfast, which isn't much, then set up the table for everyone else. I look back at the clock. 6:00. Leo should be up soon.

That's when I hear the soft padding of his feet coming down the stairs. He is a very well trained ninja, and any other person wouldn't have heard him. But I can hear everything that goes on in and outside of this home. I climb up the pole that leads to my spot and melt into the shadows. Even he can't detect me when I'm here. Or anywhere for that matter. I silently watch him as he walks into the kitchen and stops to look at what I made this morning. Now that I've been waking up before everyone else since I moved in, it's my responsibility to make breakfast. They made that rule pretty clear after they found out that I'm awake before Leo everyday.

He grabs a pancake and some bacon and sits down. I watch everyone else walk in and eat. Donatello leaves his lab last. That answers my question of whether he fell asleep or not. This is one of my favorite parts of the day. Just watching everyone at peace, thinking about who knows what.

Then I pick up Raph's voice murmuring something. Leo responds and Mikey and Donnie join in on the conversation. I have to know what they're saying. I gradually make my way down the pipe and get closer. "...we leaving," Raph asks. "I was thinking tonight. We have to hit him hard and soon before he ships those guns out," responds Leo. Donnie and Mikey nod their heads. Guess they're going to the warehouse they found two nights ago. I have a bad feeling about this. It seemed too perfect. They found it just after Donnie picked up chatter on a radio they stole.

I decide now to make my haven't spotted me or heard me or else Leo would have tensed up slightly. I walk up behind Raph and Donnie, who's backs are to me. I smile in my head. Lets see if I can scare them. I tap them on the shoulder. Donnie's throws his cup in the air, coffee flying everywhere. Raph drops his utensils and sends a punch backwards, aiming at my head. Donnie does the same. I stop both of their punches, one fist in each hand, inches from my face. Donnie realizes who it was before Raph does. He tries to drop his fist, but I'm still holding onto it, waiting for Raph to get that look out of his eye. But it doesn't leave. Guess I have to show him its me.

I drop Raph's fist and move my head aside as the next one comes sailing. He uses the other one to try and clock me, but I stick my arm up and stop it. I grab that one and crouch, using his momentum to flip him over me. He lands on his shell and groans. I stand up and walk over to him. I squat next to him and stare. I tilt my head every so slightly, using a move Mikey uses often to look innocent. Raph grumbles something incoherent. I stand and offer my hand to him. He takes it. I lean backwards, pulling him up. He stares at me for a second. I can see the anger and embarrassment in his eyes. It's silent for a few seconds before the rest of them laugh. Mikey falls out of his chair, rolling on the floor and holding his stomach. Leo and Donnie just chuckle for a longtime. "Man...she got...you good," Mikey says between his laughs.

I sit at the table with a straight face. Raph takes his seat next ot me and mutters, "Well, don't scare me next time." Leo looks at me, knowing full well I heard most of the conversation.

"Cat, I forbid you, as the leader of this team, to come on this mission. It's too dangerous."

"In case you haven't noticed Fearless, she's not technically part of the team."

"Then I forbid her as her brother."

Splinter decides to make his entrance and walks over to the table. "I heard a noise. What happened." Mikey plastered his goofy smile on his face. "Just Raph getting laid out by Cat." Raph smacked him upside the head. Splinter just stared at them. "You're late for training." Of course, Leo was the first one up and practically ran to the dojo. Everyone else got up reluctantly and slowly made their way. I didn't have to ever since I beat all of them a couple years ago. "You as well, Cat." That surprised me. I looked at him confused, but all he did was wink at me. This didn't help my nerves. But I followed him anyways.

I entered the dojo, where all my brothers were already kneeling in front of the tree. I kneeled next to Splinter and looked out at them. "I had an idea while I was meditating," Splinter began. "Cat will go up against all of you..." Groans erupted from everyone. Last time I did that, all of them had to be patched up by Donnie. I only gave them bruises last time, what are they groaning about?. "Let me finish," Splinter told them. They all shut up. "But, she will also go up against me." Leo's eyes widened. "Are you sure that's wise, Master. She's just a child."

"She may be, Leonardo. But this child defeated all of you." Leo looked down, blushing.

Splinter stood up and lined up with my brothers. They all got into a fighting stance, ready to defend themselves or initiate an attack on me. I sighed as I stood up. This is possibly the worst idea Splinter has ever had. But when I look up at everyone, that changes. The determination and cockiness in all of them is amazing. They think that just because they have Splinter on their side, they'll beat me. I get into my stance, choosing a defensive one because I know as soon as Splinter gives the go, they'll all charge at me. I smile to myself. This is going to be fun. "Ready," Splinter asks. I nod. "Begin," he shouts.

Just as I predicted, all four of them charged, except Splinter. Mikey was the first to get to me, since he's the second fastest, behind me. He swings his nunchucks at my head, trying to knock me down. He always starts with that move. I grab the nunchuk and spin, wrapping it around my arm. This yanks it out of his hand, and he yelps in surprise. I transfer it to my other hand in a second and hit him in the ribs. He yelps and stumbles away from me. That's when everyone else arrives. Raph and Leo run to my flanks and charge me from both sides. Just before they can hit me, I duck. They don't have enough time to stop themselves and run right into each other. I slide out from underneath them before they land in a heap of green and weapons. I quickly grab the sais and katanas. I throw the katanas a fair ways away from them and hold the sais. They'll be easier to use to disarm Donnie.

I barely have time to flip out of the way before Donnie's bo staff swings where my legs were. I slide down his back and land. I kick his shell before he can recover. That makes him stumble forward, but he recovers surprisingly fast. He turns around, not expecting me to be sprinting at him. He freezes which gives me enough time to put his staff between the sais and flip it out of his hands. As its sailing through the air, I throw the sais at the tree, hard enough to make them stick in the trunk and high enough that Raph will have to scale the tree in order to get them. I roundhouse Don in the plastron and catch the bo in my hands. He lands on top of the other two and I laugh internally at how they squirm, trying to get untangled.

I completely forget about Splinter until my feet are swept out from under me. I roll to recover and land a few feet away from him. I stay low and face him. His tail is swishing and circling me. I've watched him enough to see the signs that mean he's going to attack. His ears always twitch before he makes a move. When I see them do, I'm prepared for the punch that comes at me. I block it with the staff and jump over his tail going for my feet. I duck under the next punch and jab him in the stomach with Don's staff. It knocks the wind out of him and he stumbles backwards. He's still standing though. I jump over when he's not looking and hit him hard on the back, knocking him to the ground. I step over him and walk over to the heap of brother that I created.

I see Splinter charging at me from behind through the reflection in their eyes. I throw the bo out of the way and grab his arm. I flip him over me and throw him into the tree. He hits the trunk and slides down it. I grab the katanas near me and hold them at their heads. They all know that they're defeated. Splinter exhales. "Yame." Leo's the first to untangle himself and stand up. I give him back his swords and help Mikey and Raph up. Mikey walks over to pick up his nunchaku and I walk over to the trunk to receive the sais. I crouch low and jump to the lowest branch. They're stuck just below so I hang by my legs and pull them out. I sit up and hop down. I hand them to him and go sit down. Don already got his staff. I look at the clock that's in the dojo. The whole fight took 4 minutes.

All five of them kneel where they were before. I look at Splinter, who has joined them. He stares right at me. "I am very impressed, little one. Not one of my sons has been able to defeat me in combat. The speed at which you did surprised me as well. You're attacks and counters were precise and efficient." I nod a thank you towards him. "You're the master at this moment. Tell each of them what they did wrong and right, including myself." That took me aback. _I _have to tell them what they did wrong and right?

I stand up and walk over to Mikey, who's on the end. I stand in front of him, waiting for him to look up at me. He reluctantly, gazed into my eyes, his baby blues begging me to do something. I couldn't figure out what it was. It's easy to stay expressionless and non-feeling when in a fight because I don't have to talk. I step back from him and sit back down. Splinter didn't understand why for a moment, but I saw when the realization hits him.

"I apologize Catharina. I forgot that you don't like to speak. Perhaps you can whisper your observations to me and I will speak for you. I'll chat with you later on what you think of my performance." I walk over to him and whisper everything that I observed. This is the most words I've ever said at a time in my life. When I finished, Splinter and I switch spots and now it was him standing and staring at Mikey. "Now this is what Catharina observed, not me. If you have questions, ask her." He paused before continuing. "Too predictable. Next time, start with a different move." He walked down the line and looked down at Donnie. "Don't freeze. Always be moving and expect the unexpected." Next it was Raph and Leo. "Now this is possibly the best advice I've heard before. Learn to stop." Everyone chuckled. "And coordinate your attacks before engaging. Even I agree that your attack was sloppy." He walks back to stand underneath the tree. "We are finished here. You are all dismissed." They stand up and make their way out the door. Me and Splinter are the only ones left in the room. "Now, my daughter, tell me what you think of my performance." I don't speak for a few moments, just stare into his big, mousey eyes. My voice, per usual, is a whisper and barely audible. "I didn't notice anything wrong. I just caught you by surprise is all. Even I'm surprised I defeated you."

Splinter laughs and walks over to me. He kneels in front of me and takes my hands into his big ones. "I am not. You are the most talented person I have ever seen." I open my mouth to respond when I hear a noise outside the sliding doors. I whip my head towards the door, listening for more. Splinter notices and looks where I'm looking. I put my finger to my mouth, telling him to be quiet. I hear breathing. The slow inhale and exhale of someone who doesn't want to be caught. I hear four of them. I signal Splinter to stay sitting. I walk to the doors and climb up the frame. When I'm above the door, hanging onto the wood just above it by my feet, I open it. I pop my head down and look at four pairs of eyes. They all scream out of fear and fall backwards.

I'm still hanging upside down when they pick themselves up. My four brothers all looked ashamed. I glare at them with the most intense look I can give them. "Go," I tell them in a menacing tone that even scared me. They all bolt away from the door to somewhere else in the lair. I look back at Splinter, who has the biggest smile I've ever seen him make. "Good one."

I push off from the wall and landed on my feet. I made my way out the door and over to the pipe that leads to the hallway high in the wall. I climbed up it and walk passed my "room" and made my way to the other section of the pipe. I entered the giant clearing and stared out at the plants that I've grown over the last few months. I bought all the seeds from the internet on Donnie's computer after I found a book in Splinter's room about plants used by ninjas. It really interested me so I did some research and there was a shop that had all of the seeds that I knew grew in other places besides Japan. So I snuck out that night and got some. And luckily, I found the perfect place to start my garden a week earlier.

I take out the box that held the book, the jars and plugs, and some catalysts I stole from Don's lab. He won't need them anytime soon. I immediately got to work picking the leaves, pulling the plants for their roots, and every other part of the plant I could use. When I finished with that, which took me about an hour, I got to work preparing all the pieces.

I was so absorbed in my work that I didn't notice the time go by. When I finally put the last liquid in the last vial, it was almost 11. I stood up and stretched my legs and dusted the dirt off. I set aside the jars of seeds that I picked off the plants so I can grow more. That's when I heard whispers and murmurs coming from the living room. I made my way down the pipe and peered out the opening. Mikey, Leo, Donnie, and Raph were standing in a circle, preparing themselves for the raid. I reached above me and pulled myself up to the pipe that went right over them. I walked down it with my hands behind my back and listened.

When I was right on top of them, I crouched down. I could hear every word clearly.

"Everyone know the plan?" asked Leo.

"Yes, Fearless," replied Raph. "But knowing you, you'll go over it again and again on the way there."

"Shut up Raph."

When they started to leave, I jumped down and landed right in front of Leo. The speed at which they pulled out their weapons surprised me. Must be the nerves. I just stand there with my hands behind me, waiting for them to resheath. "Oh Cat..we were just...we were going to the...ah..." stuttered Mikey.

"Don't bother, she already knows," Leo told him.

"How?"

"I just do."

Leo looked back to me. "No." I leaned forward and narrowed my eyes. Asking without staying, "Why not?"

"Because I said so."

"Oh come on Leo," began Donnie. "It's just a raid. We don't expect Shredder to be there and..."

"No!"

"She beat us all, man. Even Splinter. If we get into trouble, I think having her with us would help..."

"I said no and that's final."

No one said a word. Leo kneeled down to my level and put a hand on my shoulder. I don't like it when people touch me. It makes Leo feel more brotherly, so I let him do it. "I don't want you to get hurt. If we get into trouble, I don't want to lose you. Stay here, where it's safest. Please." I give him a reluctant nod and he stands up. "Let's go, guys."

After they left the lair, Mikey stuck his head round the corner to say goodbye. When I no longer heard their footsteps, I run to the dojo and grab my fans. I've trained with these for most of my life. After, I run to the lab to grab a T-Phone so I can track them. I look around to make sure Splinter didn't hear me. That's when I saw the bandage roll out of the corner of my eye. Knowing them, they'll probably need it. I also run and grab the many vials I made and a backpack. I stop at the entrance, running through the list in my head to make sure I didn't forget anything. When I'm sure I don't need anymore, I attach the fans to my belt. At the entrance, I take one last look before leaving to help my brothers.

**See you on the next chapter (hopefully)!**


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